Graphene Oxide/Black Phosphorus Nanoflake Aerogels with Robust

Volume 9 Number 24 28 June 2017 Pages 8035–8510 Nanoscale

rsc.li/nanoscale

ISSN 2040-3372

COMMUNICATION Han Zhang et al. Graphene oxide/black phosphorus nanoﬂ ake aerogels with robust thermo-stability and signiﬁ cantly enhanced photothermal properties in air Nanoscale

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Graphene oxide/black phosphorus nanoﬂake aerogels with robust thermo-stability and Cite this: Nanoscale, 2017, 9, 8096 signiﬁcantly enhanced photothermal Received 27th January 2017, properties in air† Accepted 21st March 2017 DOI: 10.1039/c7nr00663b Chenyang Xing,a Guanghui Jing,a,b Xin Liang,a,c Meng Qiu,a Zhongjun Li,a,d a,d e a a rsc.li/nanoscale Rui Cao, Xiaojing Li, Dianyuan Fan and Han Zhang *

Here we report a new kind of three-dimensional (3D) hybrid egies including chemical reduction self-assembly and hydro- aerogels, based on graphene oxide (GO) and black phosphorus thermal reduction of GO solution, both followed by freeze- nanoflakes (BPNFs), for the first time. Our results demonstrate that drying, can readily realize the construction of 3D graphene the as-prepared GO/BPNF hybrid aerogels exhibited significantly aerogels.13,17,19,21 enhanced photothermal as well as electrical properties of GO Stimulated by successful development of graphene, investi- aerogels due to the addition of BP. Moreover, they also possessed gations into other 2D materials have been explored in recent – excellent photothermal stability under ambient conditions without years.22 30 Among those, so much attention has been paid to – any protection, which can be ascribed to the coverage of BPNFs black phosphorus (BP),31 33 also called “phosphorene”, due to with GO nanosheets in these aerogels. This exceptional photo- its thickness-tuneable bandgap (0.3 to 2.0 eV),34,35 biocompat- thermal property along with robust stability renders GO/BPNF ibility, degradability, excellent photothermal properties,36,37 aerogels with promising bio-related applications, such as photo- etc. BP, with similar but quite unique features in comparison thermal therapy for cancer treatment. with graphene, is expected to be charting a similar course to get application development. In view of the advancement of Graphene has been a two-dimensional (2D) star material since 3D graphene structures, 3D BP materials may also possess a 1 it was first mechanically exfoliated by Novoselov and Geim. larger specific surface area and porous structures, which Published on 12 April 2017. Downloaded 29/06/2017 01:52:59. The last two decades have seen tremendous achievements of – would be promising in many fields, compared with 2D BP graphene in electrics,2 6 biological engineering,7,8 environ- – materials that usually exist in the form of BP nanoflakes 9 12 – mental and energy fields. It makes still further progress (BPNFs),38 42 BP nanoparticles (BPNPs)43 and BP quantum – when it comes to 3D graphene-based materials, such as the dots (BPQDs),44 46 just like that of graphene.47 However, 13–17 known graphene aerogel and graphene fibrous unlike graphene whose oxidation state (i.e. GO) can be reduced 18–20 aerogel, with macroscopic size, microscopic opening to graphene (i.e. reduced GO) so as to build 3D graphene in holes, significantly enhanced specific surface area, excellent water, the oxidation state of BP (i.e. PxOy) is uncrystallised and electricity as well as flexibility. In conversion of 2D graphene can be directly dissolved in water into phosphoric acid, which into 3D graphene, graphene oxide (GO), also a 2D material, may be impossible to be reduced to crystalline BP either by – plays a critical role as an important precursor. Typical strat- reductive agents or by hydrothermal reduction in water.13 20 Although there is a possible difficulty to synthesize a pure BP based 3D aerogel, BP may attach itself to a 3D support/ aShenzhen Engineering Laboratory of Phosphorene and Optoelectronics, stand to form a composite 3D structure. In this regard, 3D gra- Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and phene or 3D GO may be the best choice since it has some Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, similar aspects to BP in many fields. Recently, Wan et al.48 first Shenzhen 518060, P.R. China. E-mail: [email protected] reported ultrafast gelation (1 min) of GO with poly(oxypropyl- bInstitute for Advance Study, Shenzhen University, Shenzhen 518060, P.R. China cCollege of Materials Science and Engineering, Shenzhen University, Shenzhen ene) diamine (D400) by means of covalently cross-linking to 518060, P.R. China form a 3D GO aerogel. In view of BP degradation in air, it is dFaculty of Information Technology, Macau University of Science and Technology, believed that such an ultrafast gelation of GO in water is Taipa, Macau 519020, P.R. China favourable to make BP materials attach themselves to the 3D eGraduate school at Shenzhen, Tsinghua University materials and devices testing GO support and therefore overcome the oxidation and insta- center, Shenzhen 518055, P.R. China †Electronic supplementary information (ESI) available. See DOI: 10.1039/ bility issue of BP. Inspired by these, we fabricated 3D GO/ c7nr00663b BPNF aerogels in the present study and demonstrated the first

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prototypic example of BP-related aerogels, which may arouse broader interest among other related research fields, including clean energy, optics, biological engineering, etc. GO and BPNFs were firstly prepared by the modified Hummers method and liquid exfoliation (see the Experimental section in the ESI†), respectively. Typically, the as-prepared BPNFs are of several hundred nanometers in lateral size (Fig. 1a). The clear lattice fringes of 2.23 Å assigned to the (014) plane41 and selected-area electron diﬀraction (SAED) (Fig. 1b) suggest the maintenance of BP crystal structures during preparation. Atomic force microscopy (AFM) for BPNFs in Fig. 1c and e shows a thickness range of 10–30 nm for BPNFs. In contrast, the as-prepared GO nanosheets have a larger size with several micrometers and a smaller thickness of ca. 1.5 nm in Fig. 1d and f. In a typical 3D GO/BPNF aerogel preparation, a homogeneous aqueous solution, based on GO nanosheets, BPNFs

and a cross-linking agent, poly(oxypropylene) diamine (D400), was first obtained (Fig. 2a) and then covered with aluminium foil thoroughly (Fig. 2b) in view of the degradation of BP trig- gered by light,49 followed by immersion in an oil bath at 90 °C for 1 min. After gelation of GO (Fig. 2c), a freeze-drying pro- Fig. 2 Characterization of 3D GO/BPNF aerogel with 13.4 wt% BP cedure was performed to obtain 3D GO/BPNF aerogels (Fig. 2d). content: (a) homogeneous aqueous solution containing BPNFs, GO nanosheet and D400. (b) Aluminium foil covered reaction vessel. (c) Gelation of GO and BPNFs forming a hydrogel. (d) Macroscopic view of GO/BPNF aerogel after freeze-drying treatment. Field emission scanning electron microscopy (FE-SEM) images of neat GO aerogel (e–f) and GO/ BPNF aerogel (g–h), respectively. X-ray photoelectron spectroscopy (XPS) curve of bulk BP (i), neat GO aerogel and GO/BPNF aerogel ( j), respectively.

The macroscopic shape and size of GO/BPNF aerogels are dependent on the original shape of the vessel. The mass Published on 12 April 2017. Downloaded 29/06/2017 01:52:59. content of BP in GO/BPNF aerogels was evaluated by using inductively coupled plasma-atomic emission spectroscopy (see the Experimental section in the ESI†). Neat GO aerogel without BPNFs was used as a control sample in this study. It was found that such a 3D GO/BPNFs aerogel with 13.4 wt% BP content exhibited macroporous structures at low magnification (Fig. S1, ESI†) and typical layer-stacked structures (Fig. 2g and h), which is similar to those from neat GO aerogel (Fig. 2e and f). In addition, BPNFs were not readily observed in the GO/BPNF aerogel because of their full coverage with large-sized GO nanosheets. The corresponding P element analysis by using an energy dispersive spectrometer (EDS) was also performed (Fig. S2, ESI†). In comparison with 3D graphene aerogels,16,50 such a 3D GO/BPNF aerogel has a relatively large density (11.7 ± − 0.62 mg cm 3) and weak mechanical strength (Fig. S3, ESI†). Zhou et al.49 reported the degradation principle of BP under light, oxygen and water. In this study, attention has Fig. 1 Characterization of BPNFs and GO nanosheets used in 3D GO/ been paid to the stability of BP in GO/BPNF aerogels in air. In BPNF aerogels: (a) transmission electron microscopy (TEM) image illus- the preparation procedure, GO gelation in the presence of trating the morphologies of BPNFs. (b) High-resolution TEM (HR-TEM) BPNFs was conducted in water at 90 °C, it is thus important to image showing the lattice fringes of BPNFs with an insert image of selected-area electron diﬀraction (SAED). (c) and (d) Atomic force determine whether BP was degraded in this gelation. Fig. 2i microscopy (AFM) for BPNFs and GO nanosheets, respectively. (e) and and j show the X-ray photoelectron spectra (XPS) of bulk BP (f) Height proﬁles along the lines in (c) and (d). and GO/BPNF aerogels, respectively. The bulk BP shows 2p3/2

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5+ at 129.8 eV and p at 134.2 eV. The former is attributed to the cross-linked agent D400 (detailed structural formula: H2N–[CH – – – – – – 48 zero-valent crystalline BP and the latter is assigned to oxidized (CH3) CH2 O ]6.1 CH2 CH(CH3) NH2) with and without 44,45 phosphorus (i.e. PxOy) (Fig. 2i). However, in the case of BPNFs. In Fig. 3a, GO nanosheets’ gelation occurred when the GO/BPNF aerogel (Fig. 2j), only crystalline BP signals at 130.3 adjacent GO nanosheets were linked by one and/or several 1/2 3/2 (assigned to 2p ) and 129.6 eV (assigned to 2p )are D400 chains bearing two primary amine groups in the end of observed and no oxidized phosphorus signal is found, which the chain via nucleophilic substitution reactions of the suggests that BP was not only degraded in the gelation pro- epoxide groups in GO with the primary amine groups in 48,51,52 – ’ cedure but also protected by GO nanosheets in their aerogel. D400. Thus, C N bonds were produced in GO s gelation Additionally, it is also noted that P signals in the GO/BPNF (Fig. S4 and S5, ESI†). In the presence of BPNFs, such nano- aerogel in Fig. 2j are quite weak relative to bulk BP in Fig. 2i. flakes were homogeneously dispersed within GO nanosheets This phenomenon can speculate that BPNFs were compactly (Fig. 3b). On the occurrence of GO gelation, the BPNFs were covered by several GO nanosheets (ca. 1.5 nm thickness) in instantly fixed within the gallery of GO nanosheets and this view of the detective depth of around 1–3 nm in XPS measure- intercalation of BP directly resulted in the broadening of GO ments. Fig. 3 illuminates the gelation of GO with the aid of the nanosheets with each other. Therefore, it was found that the d-spacing of GO in the GO/BPNF aerogel in Fig. 3c was larger than that of the neat GO aerogel. This finding is also support- ing the above speculation that BPNFs were covered with GO nanosheets thoroughly in Fig. 2j. Fig. 4a shows a P signal comparison of GO/BPNFs aerogel with 13.4 wt% BP before and after 30 days in air without any protection, such as an inert gas atmosphere, vacuum condition or photophobic treatment. Our finding suggested that BP in the GO/BPNFs aerogel was not oxidized by oxygen under ambient conditions after 30 days with a shifted peak at 131.2 eV and, in particular, without P signals of the oxidized state at 134–135 eV, compared to that of the fresh GO/BPNF aerogel. When the surface of 30-day-after GO/BPNF aerogel was subjected to Ar-sputtering for 60 s, only a crystalline P signal was observed, just the same as that of the fresh GO/BPNF aerogel. The above XPS results reveal that the as- prepared GO/BPNF aerogel is rather stable in air. GO has been proven to be effective in photothermal therapy – (PTT).53 57 More fascinatingly, BP has also been found to be Published on 12 April 2017. Downloaded 29/06/2017 01:52:59. promising in PTT for tumours.44,45 It is therefore reasonable to believe that GO/BPNF aerogels should have a synergistic photothermal property. Fig. 4b shows the photothermal behaviours of the neat GO aerogel and GO/BPNFs aerogel with 13.4 wt% − BP at 0.2 and 0.5 W cm 2 for 5 min in air, respectively. It is found that the neat GO aerogel has marginal photothernal pro- − perties at 0.2 W cm 2 power density and that the temperature never overpassed 40 °C after 5 min of irradiation with 808 nm near-infrared (NIR) light. This temperature was slightly increased by about 20 °C when an increased power density of − 0.5 W cm 2 was applied. In sharp contrast, in the case of GO/ BPNF aerogels with 13.4 wt% BP, a significant increment in temperature was observed. For instance, the final equilibrium temperatures of GO/BPNFs were increased by around 20 °C at − − 0.2 W cm 2 and 60 °C at 0.5 W cm 2, compared with the GO aerogel without BP. These results reveal that the addition of BP can effectively enhance the photothermal conversion of GO at low power density. It is reported that GO-based PTT for cancer is usually limited due to its low photothermal efficiency, which results in applying a larger power density (for example, 1–2 Fig. 3 Schematics for GO gelation with the aid of the cross-linked − Wcm 2) and longer irradiation time for treatment efficiency, agent D400 without (a) and with (b) BPNFs and X-ray diffraction (XRD) patterns of neat GO aerogel and GO/BPNF aerogel with 13.4 wt% BP inevitably causing additional damage to normal tissues and content (c), respectively. cells.58 It is therefore confirmed that the GO/BP aerogels in the

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the range of 100–118 °C, indicating a superior stability of the photothermal property for the as-prepared GO/BPNF aerogel. The following five cycles made this sample a slightly decreased equilibrium temperature, which may be due to slight oxidation of BP’s surface by oxygen. In spite of this, its final temperatures are still maintained at around 100 °C, which is approximately 2.7 times higher than that of the neat GO aerogel. Additionally, the dynamic rheological behaviour of such a GO/BPNF aerogel also indicated a good thermal stability in the range of 30–100 °C (Fig. S5, ESI†). A conclusion can be drawn that the GO/BPNF aerogel in this study has superior photothermal as well as thermal stability even in air at high temperatures. The eﬀect of BP loading on the photothermal and electrical properties of GO/BPNF aerogels was also investigated, as shown in Fig. 5. It shows that the final equilibrium temperatures of GO/BPNF aerogel samples increase with an increase in BP loading from 7.5–26.3 wt% BP (Fig. 5a and b). The − maximum temperature values can be up to 64 (at 0.2 W cm 2, − Fig. 5a) and 145 °C (at 0.5 W cm 2, Fig. 5b) when the BP loading was 26.3 wt%, which is about 1.8 and 2.4 times higher than those from GO aerogels without the BP. Meanwhile, these composite aerogels also show favourable photothermal stability on raising the temperature. Fig. 5c and d show typical two terminal resistor current (I)–voltage (V) curves and the corresponding resistances of GO/BPNF aerogels with various BP contents, respectively. It was found that the slope of the I–V curves of GO/BPNF aerogel samples was increasing with BP loading (Fig. 5c), indicating the improvement of the electrical properties of samples. And the corresponding resistances decreased by 1 order of magnitude when the BP loading reached 26.3 wt% in the GO/BPNF aerogel compared with the neat GO aerogel (Fig. 5d). The enhancements of both the Published on 12 April 2017. Downloaded 29/06/2017 01:52:59.

Fig. 4 Stability characterization of the as-prepared GO/BPNF aerogel with 13.4 wt% BP content. (a) XPS spectra of GO/BPNF aerogel before and after 30 days in air and after Ar sputtering for 60 s. (b) Photothermal behaviours of neat GO aerogel and GO/BPNF aerogel at a power density of 0.2 and 0.5 W cm−2 for 5 min in air. (c) Photothermal stability of such a GO/BPNF aerogel.

present study are promising for PTT in view of their excellent photothermal properties at low NIR power density. The above GO/BPNF aerogel exhibited excellent photothermal behaviour with a maximum temperature of 118 °C in air. It is of great importance to evaluate its photothermal stability because the BP is reported to be unstable under an air atmosphere.49 Fig. 4c shows the photothermal properties of GO/ BPNF aerogels with 13.4 wt% BP loading during 8 heating- Fig. 5 Photothermal behaviours (a and b) and electrical properties (c cooling process cycles. The results clearly show that the initial and d) of GO/BPNF aerogels with various BP contents, a: 0 wt% (i.e. neat 3 cycles of temperature increasing profile had no significant GO aerogel), b: 7.5 wt%, c: 9.6 wt%, d: 13.4 wt% and e: 26.3 wt%, changes and that the equilibrium temperatures were still in respectively.

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photothermal and electrical properties of GO/BPNF aerogels 15 Z. Li, Z. Liu, H. Sun and C. Gao, Chem. Rev., 2015, 115, can be ascribed to the incorporation of BP with excellent 7046–7117. photothermal properties and semiconductive feature. 16 M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, In summary, 3D GO/BPNF hybrid aerogels were reported in J. H. Satcher Jr. and T. F. Baumann, J. Am. Chem. Soc., this study for the first time. It was found that, compared to 2010, 132, 14067–14069. that of the neat GO aerogel, the as-prepared GO/BPNF aerogels 17 Z. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Müllen, exhibited significantly enhanced photothermal properties and J. Am. Chem. Soc., 2012, 134, 9082–9085. electrical properties due to the incorporation of BP. Moreover, 18 Z. Xu, Y. Zhang, P. Li and C. Gao, ACS Nano, 2012, 6, 7103– the BPNFs in these aerogels were covered by GO nanosheets, 7113. which led to a robust photothermal stability under ambient 19 J. H. Li, J. Y. Li, L. 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